1. Field of the Invention
The present invention relates to an iron-based rare-earth alloy powder, which can be used effectively as a material for a bonded magnet, and a method of making the alloy powder. The present invention also relates to a bonded magnet made from the rare-earth alloy powder and further relates to various types of electric equipment including the bonded magnet.
2. Description of the Related Art
A bonded magnet is currently used in various types of electric equipment including motors, actuators, loudspeakers, meters and focus convergence rings. A bonded magnet is a magnet obtained by mixing together a magnet powder and a binder (such as a rubber or a resin) and then compacting and setting the mixture.
An iron-based rare-earth alloy (e.g., Fe-R-B based, in particular) nanocomposite magnet has recently been used more and more often as a magnet powder for a bonded magnet because such a magnet powder is relatively cost effective. The Fe-R-B based nanocomposite magnet is an iron-based alloy permanent magnet in which nanometer-scale crystals of iron-based borides (e.g., Fe3B, Fe23B6 and other soft magnetic phases) and those of an R2Fe14B phase as a hard magnetic phase are distributed uniformly within the same metal structure and are magnetically coupled together via exchange interactions.
The nanocomposite magnet includes soft magnetic phases and yet exhibits excellent magnet performance due to the magnetic coupling between the soft and hard magnetic phases. Also, since there are those soft magnetic phases including no rare-earth elements R such as Nd, the total percentage of the rare-earth elements R can be relatively low. This is advantageous for the purposes of reducing the manufacturing cost of magnets and supplying the magnets constantly. Furthermore, since the magnet includes no R-rich phases in the grain boundary, the magnet also excels in anticorrosiveness.
Such a nanocomposite magnet is obtained by solidifying a molten material alloy (i.e., xe2x80x9cmolten alloyxe2x80x9d) by a rapid cooling process and then subjecting the rapidly solidified alloy to an appropriate heat treatment process. A single roller method is often used to rapidly cool the molten alloy. The single roller method is a method of cooling and solidifying a molten alloy by bringing the alloy into contact with a rotating chill roller. In this method, the resultant rapidly solidified alloy has the shape of a thin strip (or ribbon), which is elongated in the peripheral velocity direction of the chill roller. This method of rapidly cooling a molten alloy by bringing the alloy into contact with the surface of a solid is called a xe2x80x9cmelt-quenching processxe2x80x9d.
On the other hand, in preparing a conventional extensively used powder for a bonded magnet, a rapidly solidified alloy thin strip with a thickness of 50 xcexcm or less (typically about 20 xcexcm to about 40 xcexcm) is obtained at a roller surface peripheral velocity of 15 m/s or more The rapidly solidified alloy thin strip obtained in this manner is thermally treated and then pulverized to a mean particle size of 300 xcexcm or less (typically about 150 xcexcm) to be a rare-earth alloy powder for a permanent magnet. The particles of the rare-earth alloy powder obtained in this manner have a flat shape and have aspect ratios that are less than 0.3. As used herein, the xe2x80x9caspect ratioxe2x80x9d means the ratio of the minor-axis size of a powder particle to the major-axis size thereof. The rare-earth alloy powder or magnet powder obtained by the melt-quenching process described above will be simply referred to herein as a xe2x80x9cconventional rapidly solidified rare-earth alloy powderxe2x80x9d or a xe2x80x9cconventional rapidly solidified magnet powderxe2x80x9d. An Fe-R-B based MQ powder available from Magnequench International Inc. (which will be referred to herein as xe2x80x9cMQI Inc.xe2x80x9d) is widely known as a typical conventional rapidly solidified magnet powder.
By mixing the conventional rapidly solidified rare-earth alloy powder with a resin (or rubber), a compound to make a magnet (which will be simply referred to herein as a xe2x80x9ccompoundxe2x80x9d) is prepared. An additive such as a lubricant is sometimes mixed with this compound. Thereafter, by compacting the resultant compound into a desired shape by a compression, extrusion or injection molding process, for example, and then by magnetizing the compact, a bonded magnet is obtained as a compact for a permanent magnet (which will be sometimes referred to herein as a xe2x80x9cpermanent magnet bodyxe2x80x9d). It should be noted that a rare-earth alloy powder to exhibit desired permanent magnet performance when magnetized or a magnetized rare-earth alloy powder will be sometimes referred to herein as a xe2x80x9cpermanent magnet powderxe2x80x9d or simply xe2x80x9cmagnet powder (or magnetic powderxe2x80x9d.
The conventional rapidly solidified magnet powder has a flat particle shape as described above. Accordingly, a compound obtained by mixing the conventional rapidly solidified magnet powder with a resin (or rubber) powder exhibits poor flowability or packability during the compaction process thereof. To achieve flowability that is high enough to perform the compaction process smoothly, the percentage of the resin or rubber may be increased. In that case, however, the magnet powder percentage is limited. Or only limited compaction methods and/or compact shapes are available to compact such a material with poor flowability.
Recently, as various types of electric equipment have further reduced their sizes and further improved their performance, it has become more and more necessary to make magnets having an even smaller size and even higher performance. For that purpose, there is a growing demand for a compound that exhibits so high flowability as to fill even a small gap (e.g., with a width of about 2 mm) just as intended. For example, as in an IPM (interior permanent magnet) type motor including a magnet embedded rotor as disclosed in Japanese Laid-Open Publication No. 11-206075, a demand for a compound with high flowability goes on increasing.
Also, when the conventional rapidly solidified magnet powder is used, the magnet powder percentage (i.e., the ratio of the volume of magnet powder to that of overall bonded magnet) is at most about 80% when the powder is compacted by compression and at most about 65% when the powder is compacted by injection molding. The magnet powder percentage will determine the performance of permanent magnets as final products. Thus, to improve the performance of permanent magnets, the magnet powder percentage is preferably increased.
To increase the flowability of the conventional rapidly solidified magnet powder, Japanese Laid-Open Publication No. 5-315174 proposes a method in which a magnet powder obtained by a gas atomization process is used. According to this publication, the magnet powder prepared by the gas atomization process has almost granular particles. Thus, by adding this magnet powder to the conventional rapidly solidified magnet powder, the flowability can be increased. However, it is difficult to make a magnet powder exhibiting sufficient magnetic properties by a gas atomization process. Thus, this method is far from being an industrially applicable method. The reason is as follows. Specifically, the gas atomization process results in a lower cooling rate than the melt-quenching process described above. Accordingly, only very fine particles can satisfy the rapid cooling conditions that should be met to obtain particles with sufficient magnetic properties. Also, a melt of the rare-earth alloy having the composition disclosed in the publication identified above has a relatively high viscosity. Thus, it is hard to obtain fine particles. Consequently, according to the method disclosed in the publication identified above, the yield of those fine particles having sufficient magnetic properties is very low and the productivity is also very bad because a classification process step must be carried out to obtain particles with a desired particle size.
In order to overcome the problems described above, a primary object of the present invention is to provide a compound of which the flowability is improved by controlling the particle size distribution of an iron-based rare-earth alloy powder for use to make a bonded magnet, and provide such an iron-based rare-earth alloy powder.
Another object of the present invention is to provide a bonded magnet, which can exhibit excellent permanent magnet performance, by using the compound and by increasing the flowability and/or the magnet powder percentage, and an electric appliance including such a bonded magnet.
An iron-based rare-earth alloy powder according to the present invention includes: a first iron-based rare-earth alloy powder, which has a mean particle size of 10 xcexcm to 70 xcexcm and of which the powder particles have aspect ratios of 0.4 to 1.0; and a second iron-based rare-earth alloy powder, which has a mean particle size of 70 xcexcm to 300 xcexcm and of which the powder particles have aspect ratios of less than 0.3. The first and second iron-based rare-earth alloy powders are mixed at a volume ratio of 1:49 to 4:1, whereby the objects described above are achieved.
In a preferred embodiment, the first iron-based rare-earth alloy powder has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb; and the mole fractions x, y, and z satisfy the inequalities of: 10 at %xe2x89xa6xxe2x89xa630 at %; 2 at %xe2x89xa6y less than 10 at %; 0 at %xe2x89xa6zxe2x89xa610 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively.
The first iron-based rare-earth alloy powder preferably includes, as its constituent phases, an Fe phase, an FeB compound phase and a compound phase having an R2Fe14B-type crystalline structure, and the respective constituent phases preferably have an average crystal grain size of 150 nm or less.
In another preferred embodiment, the first iron-based rare-earth alloy powder has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb and always includes Ti; and the mole fractions x, y, z and m satisfy the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at % less than y less than 10 at %; 0.1 at %:xe2x89xa6zxe2x89xa612 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively. The percentage of Ti to the overall element M is preferably at least 60 at %, more preferably 80 at % or more.
The first iron-based rare-earth alloy powder preferably includes at least two ferromagnetic crystalline phases, of which hard magnetic phases preferably have an average crystal grain size of 5 nm to 200 nm and soft magnetic phases preferably have an average crystal grain size of 1 nm to 100 nm. More preferably, the average crystal grain size of the hard magnetic phases is greater than that of the soft magnetic phases.
The second iron-based rare-earth alloy powder preferably has a composition represented by the general formula: (Fe100-x-yQxRy, where Fe is iron; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; and the mole fractions x and y satisfy the inequalities of 1 at %xe2x89xa6xxe2x89xa66 at % and 10 at %xe2x89xa6yxe2x89xa625 at %, respectively.
A method of making an iron-based rare-earth alloy powder according to the present invention includes the steps of: (a) providing a first iron-based rare-earth alloy powder, which has a mean particle size of 10 xcexcm to 70 xcexcm and of which the powder particles have aspect ratios of 0.4 to 1.0; (b) providing a second iron-based rare-earth alloy powder, which has a mean particle size of 70 xcexcm to 300 xcexcm and of which the powder particles have aspect ratios of less than 0.3; and (c) mixing the first and second iron-based rare-earth alloy powders at a volume ratio of 1:49 to 4:1, whereby the objects described above are achieved.
In a preferred embodiment, the first iron-based rare-earth alloy powder has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb; and the mole fractions x, y, and z satisfy the inequalities of: 10 at %xe2x89xa6xxe2x89xa630 at %; 2 at %xe2x89xa6y less than 10 at %; 0 at %xe2x89xa6zxe2x89xa610 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively.
In another preferred embodiment, the first iron-based rare-earth alloy powder has a composition represented by the general formula: (Fe1-mTm)100-x-y-zQxRyMz, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; M is at least one element selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb and always includes Ti; and the mole fractions x, y, z and m satisfy the inequalities of: 10 at % less than xxe2x89xa620 at %; 6 at % less than y less than 10 at %; 0.1 at %xe2x89xa6zxe2x89xa612 at %; and 0xe2x89xa6mxe2x89xa60.5, respectively.
The step (a) preferably includes the steps of: cooling a melt of the first iron-based rare-earth alloy by a melt-quenching process, thereby forming a rapidly solidified alloy with a thickness of 70 xcexcm to 300 xcexcm; and pulverizing the rapidly solidified alloy.
The method may further include the step of thermally treating and crystallizing the rapidly solidified alloy before the step of pulverizing is performed.
The step of pulverizing is preferably carried out with a pin mill machine or a hammer mill machine.
The rapidly solidified alloy preferably includes at least one metastable phase, which is selected from the group consisting of Fe23B6, Fe3B, R2Fe14B and R2Fe23B phases, and/or an amorphous phase.
The step of cooling preferably includes the step of bringing the melt into contact with a roller, which is rotating at a roller surface peripheral velocity of 1 m/s to 13 m/s, thereby forming the rapidly solidified alloy.
The step of cooling is preferably carried out within a reduced-pressure atmosphere.
The reduced-pressure atmosphere preferably has an absolute pressure of 1.3 kPa to 90 kPa.
The second iron-based rare-earth alloy powder preferably has a composition represented by the general formula: (Fe100-x-yQxRy, where Fe is iron; Q is at least one element selected from the group consisting of B and C and always includes B; R is at least one rare-earth element selected from the group consisting of Pr, Nd, Dy and Tb; and the mole fractions x and y satisfy the inequalities of 1 at %xe2x89xa6xxe2x89xa66 at % and 10 at %xe2x89xa6yxe2x89xa625 at %, respectively.
A compound for use to make a magnet according to the present invention includes the iron-based rare-earth alloy powder according to any of the preferred embodiments of the present invention described above and a resin, whereby the objects described above are achieved. The resin is preferably a thermoplastic resin.
A permanent magnet according to the present invention is made of the compound according to any of the preferred embodiments of the present invention described above. A permanent magnet having a density of at least 4.5 g/cm3 can be obtained. Furthermore, a permanent magnet having a density of 5.5 g/cm3 or more, or even 6.0 g/cm3 or more, can also be obtained.
A method of making a compound for use to make a magnet according to the present invention includes the steps of: preparing the iron-based rare-earth alloy powder by the method according to any of the preferred embodiments of the present invention described above; and mixing the iron-based rare-earth alloy powder and a resin together.
The resin is preferably a thermoplastic resin.
A method for producing a permanent magnet according to the present invention preferably includes the step of injection-molding the compound made by the method described above.
A motor according to the present invention includes: a rotor including the permanent magnet according to any of the preferred embodiments of the present invention described above; and a stator, which is provided so as to surround the rotor.
A method for fabricating a motor according to the present invention includes the steps of: preparing a rotor, which has a magnet slot in its iron core; injection-molding the above-described compound for use to make a magnet in the magnet slot; and providing a stator that surrounds the rotor.